Blends of polyethylene terephthalate and poly(1,4-butylene terephthalate) are known in the art. See, for example, Fox et al, U.S. Pat. No. 3,953,394, which discloses blended compositions comprising a combination of (a) a poly(ethylene terephthalate) resin and (b) a poly(1,4-butylene terephthalate) resin. The alloyed combination of resins can also include reinforcing agents and flame retardants.
A common problem with flame-retarded blends of polyethylene terephthalate and polybutylene terephthalate is the transesterification reaction between polyethylene terephthalate and polybutylene terephthalate. The transesterification reaction occurs to a great extent when the blend is held for a period of time at processing temperatures in the presence of antimony trioxide (Sb.sub.2 O.sub.3) flame retardant. As a result of the transesterification reaction promoted by the Sb.sub.2 O.sub.3, a copolymer is formed which exhibits a lower heat deflection temperature than that of blends of polyethylene terephthalate and polybutylene terephthalate which do not contain Sb.sub.2 O.sub.3. If transesterification is suppressed, the heat deflection temperatures of non-burning, non-dripping blends of polyethylene terephthalate and polybutylene terephthalate are about the same as those of non-burning, non-dripping polybutylene terephthalate products. Heat deflection temperature can be measured by ASTM procedure D648.
The occurrence of transesterification is also indicated by changes in the melting temperature of the polybutylene terephthalate component of a polyester blend composition. More specifically, transesterification is indicated by a decrease in the temperature corresponding to the endotherm associated with the melting temperature of polybutylene terephthalate, when measured by differential scanning calorimetry (DSC). The effect on the melting temperature of the PET component is not a reliable indicator of the occurrence of transesterification.
A decrease in the melt crystallization temperature of a PET/PBT blend composition can also be indicative of transesterification. However, this effect can be due to other factors, such as the presence of nucleating agents, etc.
It has now been found that the transesterification reaction and the disadvantageous results thereof in non-burning, non-dripping blends of polyethylene terephthalate and polybutylene terephthalate are substantially reduced or eliminated by the inclusion of a particular flame retardant which may be substituted for or used in combination with the antimony trioxide which is commonly employed as a flame retardant in non-burning, non-dripping blends of polyethylene terephthalate and polybutylene terephthalate.
The particular flame retardant which is employed in the composition of the present invention is the amorphous non-abrasive reaction product of antimony trioxide, or a compound of antimony which generates antimony trioxide, and a substrate comprising a hydrous compound of silicon, as disclosed in white et al, U.S. Pat. No. 3,740,245. White et al discloses reacting a trivalent compound of antimony with a substrate comprising a hydrous compound of silicon at a weight ratio of approximately 1:1 to 1:4 on an antimony trioxide:substrate basis at a critical temperature in the range of from 400.degree. C. to 500.degree. C. in an oxygen containing atmosphere. The reaction product is a particulate solid which is non-abrasive and which is useful as a flame retardant additive in plastics, such as polyvinyl chloride, epoxy resin, polyesters, rubber, and the like. The patent does not disclose the effectiveness of the product in suppressing transesterification in blends of polyethylene terephthalate and polybutylene terephthalate.
A similar disclosure is found in the article "Novel Antimony Based Flame Retardants," by Hans Hansen, Robert Hunter, and Joseph Schwarcz, Pigments and Chemicals Division, NL Industries, Inc. (Highstown, New Jersey). The article further discloses that the antimony product discussed therein is more active than antimony trioxide and, when the product is used as a flame retardant, a 50% reduction in antimony concentration is possible as result of using antimony in the more active form.
U.S. Pat. No. 4,048,135 discloses thermoplastic linear saturated polyesters containing flameproofing agents. The flameproofing agents consist of halogen compounds and a supported antimony compound which is prepared by precipitating antimony trioxide from an aqueous suspension onto an inorganic support. It is disclosed that the use of antimony trioxide is known to have a degrading effect on linear polyesters and that the use of antimony trioxide obtained in the disclosed manner requires at most half the amount of antimony trioxide required in conventional systems. The preferred linear saturated polyester is polybutylene terephthalate. Again, there is no disclosure of the use of an antimony trioxide/talc reaction product to suppress transesterification in blends of polyethylene terephthalate and polybutylene terephthalate.
U.S. Pat. No. 4,111,892 discloses a reinforced fire retardant polytetramethylene terephthalate resin composition having improved electrical characteristics. The polytetramethylene terephthalate resin may be a mixture of at least 60% by weight of a polytetramethylene terephthalate and up to 40% by weight of at least one other polymer, such as polyethylene terephthalate. The composition further includes a halogen compound and antimony compounds as a fire retardant combination, a combination of hydrated silica and talc in a specified ratio, and talc and glass fibers as reinforcing fillers. Again, there is no disclosure of the inclusion of an antimony trioxide/talc reaction product to suppress transesterification in blends of polyethylene terephthalate and polybutylene terephthalate.
U.S. Pat. No. 4,140,669 discloses a polyester blend composition containing a talc/silica combination which provides improved heat deflection temperature under load in molded articles.
U.S. Pat. No. 3,624,024 and U.S. Pat. No. 3,963,669 disclose polyester resin compositions containing polybutylene terephthalate homopolymers or copolymers, glass fibers, flame retardants, and talc. These patents do not relate to blends of polybutylene terephthalate and polyethylene terephthalate.
U.S. Pat. No. 4,035,333 relates to a flame resistant resin composition having improved arc resistance comprising polytetramethylene terephthalate, a flame retardant halogen compound, sodium antimonate or a sodium antimonate-antimony trioxide mixture, and, optionally, talc.
U.S. Pat. No. 3,671,487 discloses the inclusion of polytetrafluoroethylene resin in a normally flammable linear polyester in order to control dripping. U.S. Pat. No. 3,962,174 discloses the inclusion of a thermoplastic phenoxy resin in polybutylene terephthalate or polypropylene terephthalate polymers or copolymers.
It can be seen from the review of the prior art provided above that there has been no disclosure of the inclusion of the reaction product of talc and antimony trioxide or an antimony trioxide generating compound in blends of polyethylene terephthalate and polybutylene terephthalate in order to suppress transesterification.
Therefore, it is an object of the present invention to provide a non-burning, non-dripping polyester blend composition in which transesterification is at least partially suppressed.
It is also an object of the present invention to provide a non-burning, non-dripping polyester blend composition which includes the amorphous non-abrasive reaction product of antimony trioxide or a compound of antimony which generates antimony trioxide and a substrate comprising a hydrous compound of silicon, the composition exhibiting suppression of transesterification, and therefore also improved heat deflection temperature over conventional non-burning, non-dripping polyester blend compositions.
It is also an object of the present invention to provide a low-cost, non-burning, non-dripping polyester blend composition which exhibits a heat deflection temperature comparable to that of polybutylene terephthalate compositions, due to at least partial suppression of transesterification.
It is also an object of the present invention to provide a method for suppressing transesterification, and thereby improving heat deflection temperature, in non-burning, non-dripping polyester blend compositions by at least partially replacing a conventional synergistic flame retardant consisting essentially of antimony trioxide with the amorphous non-abrasive reaction product of antimony trioxide or a compound of antimony which generates antimony trioxide and a substrate comprising a hydrous compound of silicon.
These and other objects, as well as the scope, nature, and utilization of the present invention, will be apparent to those skilled in the art from the following detailed description.